144 THE FLOWERING PROCESS 



and the results were shown in Fig. 7-5. Saturation near the middle 

 of the dark period does not measure the amount of pigment (flowering 

 is completely inhibited before pigment saturation is reached), but in 

 various experiments (not all shown) approximately the same quantity 

 of light was required to saturate the pigment system at any time 

 between 2 or 3 hr after the beginning of the dark period and about 

 7 hr, when complete inhibition is obtained. Thus pigment conversion 

 must be complete within the first 2 or 3 hr, and a timing mechanism 

 based upon pigment conversion will not account for the critical dark 

 period. 



In the early stages of work on the reversible pigment system, Harry 

 Borthwick and his co-workers at Beltsville were able to shorten the 

 critical dark period by preceding it with far-red light. But they were 

 only able to shorten it 2 or 3 hr. This is consistent with the idea that 

 the critical dark period is measured only for the first two or three 

 hours by conversion of the phytochrome system, and then some 

 other time-measuring reaction must take over (see also the tempera- 

 ture experiments in Section 7 below and in the next chapter). Even 

 before phytochrome was known, Withrow and his co-workers had 

 shown with other plants that maximum sensitivity to light was 

 reached within two or three hours of darkness, another indication 

 that the maximum amount of R-phytochrome had been obtained by 

 that time. 



The indications are that a maximum amount of R-phytochrome 

 has accumulated after a few hours of darkness, but that the sensitivity 

 of the flowering process to F-phytochrome increases continually as 

 the dark period progresses (according to some oscillating timer?). 

 Just before the critical night, only about 30% of the potential 

 amount of F-phytochrome is required to inhibit the flowering process 

 completely (Fig. 7-5). 



6. The Critical Dark Period under Threshold Light Conditions 



While writing the above paragraphs, it became apparent that an 

 important experiment had to be performed. The critical night should 

 be determined under threshold light conditions. Such conditions 

 imply that the dark conversion to R-phytochrome is not allowed to 

 proceed as far as under conditions of total darkness. An equilibrium 

 is set up in which the tendency to convert to R-phytochrome is 

 balanced by the formation of F-phytochrome by low intensity light. 



